The present invention relates to the improved structure of a print head for a dot matrix printer and, in particular, relates to the structure of the print head of a serial printer which can operate with improved high speed operation.
FIG. 1 shows the principle of dot matrix printing in a serial printer. A printer head 100 has seven needles for mosaic or dot matrix printing, and travels along a printing line in the direction of the arrow A. During travelling, needles are selectively driven to strike a paper through an ink ribbon and a desired pattern "A", "B", "C" or "D" is printed. The selection of needles is controlled by the content of an integrated circuit (IC) memory. When the size of a character to be printed is 2.67 mm.times.2.05 mm, a 7.times.5 dot matrix is large enough for printing a recognizable character.
One of the prior needle dot print heads for a dot printing process is shown in the U.S. Pat. No. 3,896,918, in which an electro magnetic drive structure for the operation of print needles of a mosaic printing head includes a pivotally mounted armature for each needle arranged along a circular arc. The construction includes a common yoke for all of the electro magnets which comprises two concentric cups or walls forming a single unit with cylindrical cores arranged at equal intervals along a circular arc parallel to the genatrix of the cup and located between the indivudual yoke cups. However, said prior print head has the disadvantages that the power consumption for driving the needles is large, the size of the apparatus is large, and the operational speed of the printer is rather slow. These disadvantages result mainly from the fact that a needle is driven by an electromagnet, and all the printing energy for striking a paper by a needle is given by said electromagnet.
Another print head for a serial dot matrix printer is shown in U.S. Pat. No. 4,225,250, in which a print needle is biased to a first position by a permanent magnet, and balanced at that first position with the force of a spring. When an electromagnet is energized, the flux of the permanent magnet is cancelled, and the needle is moved to a second position by the force of the spring. In this prior art device, the printing energy of the needle for striking the paper is produced by a spring, but not by an electromagnet. Therefore, this printer can be small in size, lower in power consumption, and operate with a relatively high printing speed. However, this printer head has the disadvantage that the printing speed is still not quick enough. In our experiments, this type of print head can operate with a printing speed of 1500 dots per second, but the operational speed of 3000 dots per second is desired.
U.S. Pat. No. 3,955,049 discloses another type of printer head, but the operational speed of this printer is still not quick enough.
The structure of the main part of a typical prior print head is shown in FIGS. 2A and 2B, in which FIG. 2A is a plan view and FIG. 2B is a side view, and only a single needle and the related magnet are shown for the sake of the simplicity of the drawing although an actual print head has a plurality of needles. In these figures, the yoke 5, the permanent magnet 4, the core 6, the electromagnet 3, and the armature 1 form the substantially closed magnetic path, and the armature which has a print needle 7 at the extreme end thereof is supported by the leaf spring 2, the end of which is fixed to the yoke 5 at the point P as shown in the drawings. When the electromagnet 3 is not energized, the armature 1 is attracted to the core 6 by the flux generated by the permanent magnet 4 in the closed magnetic path, and the spring 2 is curved and stores energy. Next, when the electromagnet is energized, the flux generated by the electromagnet cancels the flux of the permanent magnet 4, and thus, the net flux is not sufficient to attract the armature 1. Then, the armature 1 is released and leaves the top of the core 6, and the print needle 7 at the extreme end of the armature 1 is urged to move in the direction of the arrow, to strike the paper and print a dot. However, it should be noted that the armature 1, the spring 2 and the print needle 7 form a bulk moving body with a rotational center near the point P which is the contact point of the leaf spring 2 and the yoke 5, and that the length between the rotational center and the center of gravity .tau. of said moving body is rather long. In this situation, when the print needle 7 strikes a paper, the moving body still has energy, and the center of gravity of the moving body still moves by inertia, thus the moving body vibrates for a while after each strike action of the print needle. The vibration of the moving body causes the vibration of a print needle.
FIG. 3 shows the vibration of a print needle in a prior art printer head in which the horizontal axis shows time, and the vertical axis shows the displacement of the tip of a print needle. In FIG. 3, a print needle strikes a paper at the time T, but after striking it vibrates as shown in FIG. 3, and when the amplitude of the vibration is large, the needle strikes the paper a second time. This vibration of the print needle increases substantially the contact time of a print needle with a paper, and the energy stored in the spring or the moving body is released very slowly.
Accordingly, the power of impact or striking by a print needle on a paper is rather small as compared with the energy stored in the leaf spring or the kinetic energy of the moving body. The small impact power causes a reduction of the darkness of the printed dot, and a decrease in the printing speed, since the print needle restores slowly because of the small impact force. Further, the vibration of the spring causes the leaf unstable operation of the printer head.
That disadvantage of the vibration might be overcome by using a cross shaped spring instead of a leaf spring, so that the cross shaped spring does not become deformed by the reaction of the impact. However, the cross shaped spring is complicated in structure, as it has two leaf springs crossed with each other, and those two leaf springs must be fixed to the armature and the yoke. Thus, the manufacturing cost of the printer head with a cross shaped spring would be high.